With reference to the telecommunications sector and, in particular, to the mobile phone sector, the use is known of communication systems for both the indoor and outdoor distribution of one or more radio-frequency signals.
The known systems of communication are usable, in particular, to carry out the so-called remotization of the signals coming from one or more base radio stations (Base Transceiver Station) or from similar sets, for the outdoor coverage of signals both within a single frequency band and within several frequency bands and/or for different operators.
The use is further known of so-called DAS (Distributed Antenna System) communication systems for the distribution of signals within buildings, underground railways, airports and, in general, inside all those areas in which it is hard to guarantee adequate radio coverage using conventional methods.
The known communication systems are generally composed of one or more remote units, suitably installed in the proximity of an area in which radio coverage is to be provided, and of a main unit connected to the remote units by means of a communication channel.
Generally speaking, the remote units are provided with respective antennas for the transmission of signals or, in the case of indoor type systems, are connected to a passive signal distribution network.
With particular reference to the remotization of radio-frequency signals, a communication system of known type commonly comprises:                at least a communication channel in optical fibre connecting the main unit to at least a corresponding remote unit;        a main unit interfaced with one or more BTS stations or similar sets and suitable for converting the signals coming from the receiver into corresponding optical signals, or vice versa;        one or more remote units, each of which suitable for converting the optical signals into corresponding electrical signals, or vice versa, and amplifying such electrical signals, before sending them towards a transmission antenna.        
In use, first of all, each of the BTS stations present generates a respective signal, within a specific frequency band and for a specific operator, subsequently sent to the main unit.
The main unit converts such electrical signals received into corresponding optical signals and sends them towards a corresponding remote unit through the optical fibre communication channel.
The transmission of the optical signals between the main unit and the remote unit can be made both by means of a modulation of analogue type and by means of a modulation of digital type.
In the first case, the intensity of the optical signal is modulated according to the electrical signal to be transmitted.
In the second case, the main unit performs a conversion from analogue to digital of the electrical signals to be transmitted, before sending these by means of the optical fibre or, alternatively, the electrical signals to be transmitted are delivered by the BTS station to the main unit already in digital format.
The remote unit converts the optical signals received into corresponding electrical signals, which are then amplified and sent to a transmission antenna.
The known radio-frequency signal distribution systems have, however, several drawbacks.
In particular, the analogue transmission of the signals from the main unit towards the remote units and, subsequently, towards the antennas, does not allow an independent control of the signals.
This aspect is definitely of considerable importance within the ambit of mobile phones, whenever the need or the demand exists for independent antennas usable for different operators and/or frequency bands; in this case the different signals transmitted must necessarily be made available for the different operators for distribution to the end consumers.
Digital transmission, on the other hand, presents the technical limit of the maximum sampling frequency and, consequently of the band available for the transmission of signals on the optical fibre.
In this case, therefore, the available band is restricted and this makes the system suitable for remotizing signals contained in a narrow frequency band and usable typically in the case of mobile phone application, for just one operator.
Document US 2004/0110534 discloses capacity enhancement schemes for forward and reverse links of distributed cellular base stations consisting of a central unit and one or more remote units.
However, also these known solutions present technical limits.